U.S. patent application number 11/756977 was filed with the patent office on 2012-05-10 for atmospheric pressure charge-exchange analyte ionization.
This patent application is currently assigned to JEOL USA, INC.. Invention is credited to Robert B. Cody.
Application Number | 20120112051 11/756977 |
Document ID | / |
Family ID | 40229391 |
Filed Date | 2012-05-10 |
United States Patent
Application |
20120112051 |
Kind Code |
A1 |
Cody; Robert B. |
May 10, 2012 |
Atmospheric Pressure Charge-Exchange Analyte Ionization
Abstract
A non-radioactive atmospheric pressure method for ionization of
analytes comprises creating an electrical discharge in a carrier
gas thus creating metastable neutral excited-state species. The
carrier gas is directed at the analytes and the analytes under
conditions to suppress protonated water and water clusters.
Inventors: |
Cody; Robert B.;
(Portsmouth, NH) |
Assignee: |
JEOL USA, INC.
Peabody
MA
|
Family ID: |
40229391 |
Appl. No.: |
11/756977 |
Filed: |
June 1, 2007 |
Current U.S.
Class: |
250/282 |
Current CPC
Class: |
H01J 49/145
20130101 |
Class at
Publication: |
250/282 |
International
Class: |
H01J 49/26 20060101
H01J049/26 |
Claims
1. Method of producing analyte, analyte fragment and/or analyte
adduct ions for mass spectrographic analysis comprising the steps
of: introducing a carrier gas at atmospheric pressure into a
chamber, adding energy to the chamber creating metastable neutral
excited-state species; and directing the carrier-gas metastable
neutral excited-state species mixture into contact with the analyte
maintained at atmospheric pressure and near ground potential under
conditions that suppress the formation of protonated water
clusters.
2. A mass spectrometry method comprising the steps of: introducing
a carrier gas at atmospheric pressure into a chamber; adding energy
to the chamber creating metastable neutral excited-state species;
directing the carrier gas metastable neutral excited-state species
mixture into contact with the analyte maintained at atmospheric
pressure and near ground potential under conditions that minimize
the formation of protonated water clusters to form analyte, analyte
fragment and/or analyte adduct ions directly or via an intermediate
reactant gas; and directing analyte, analyte fragment and/or
analyte adduct ions into a mass spectrometer.
3. The method according to claim 2, wherein the atmosphere in the
vicinity of the analyte is swept with a low-humidity gas.
4. The method according to claim 2, wherein the atmosphere in the
vicinity of the analyte is swept with pure oxygen.
5. The method according to claim 3, wherein the analyte is placed
within 5 mm of the sampling orifice of the mass spectrometer.
6. The method according to claim 2, wherein a grid beyond the
chamber is set to a potential of at least 500 volts.
7. The method according to claim 2, wherein the carrier gas
consists substantially entirely of one or more of nitrogen and
noble gases with an available metastable state high enough to
ionize the analyte directly or via an intermediate reactant
gas.
8. The method according to claim 7, wherein the intermediate
reactant gas is oxygen.
9. The method according to claim 7, wherein the intermediate
reactant gas is nitrogen.
10. The method according to claim 7, wherein the intermediate
reactant gas is fluorobenzene.
11. The method according to claim 7, wherein the intermediate
reactant gas is anisole.
12. The method according to claim 2, wherein the carrier gas is
heated to promote fragmentation as well as formation of molecular
ions.
13. The method according to claim 1 or 2, comprising establishing a
potential difference in the chamber for adding energy to the
carrier gas to create metastable neutral excited-state species.
14. The method according to claim 1 or 2, comprising using photo
excitation for adding energy to the carrier gas to create
metastable neutral excited-state species.
15. The method according to claim 1 or 2, comprising using
microwaves for adding energy to the carrier gas to create
metastable neutral excited-state species.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to atmospheric ionization of
analytes and mass spectrometric methods.
[0003] 2. Description of Related Art
[0004] A method of analyte detection which is capable of detecting
trace analytes on surfaces at atmospheric pressure through the use
of metastable neutral excited-state species or ionized derivatives
thereof is described in U.S. Pat. No. 6,949,741 entitled
"Atmospheric Pressure Ion Source" and U.S. Pat. No. 7,112,785
entitled "Method for Atmospheric Pressure Analyte Ionization."
These methods enable sampling neutral analyte molecules without the
restriction of relocating the analyte from the surfaces on which
they are attached. For example, cocaine from cash currency and
chemical/biological warfare agents from surfaces of military
interest can be sampled directly and in situ without swabbing or
solvent washing the surface.
[0005] This method is normally operated under conditions wherein
the primary mode of ionization of the analyte is proton transfer
from ionized water clusters. Under these conditions, the largest
peaks in the background spectrum are water clusters
[(H.sub.2O).sub.n+H].sup.+ formed by interaction of excited-state
helium atoms with atmospheric moisture.
[0006] Water has a proton affinity (PA) of 691 kJ/mol. Proton
transfer occurs if the analyte has a higher proton affinity than
the proton affinity of water clusters according to the following
chemical equation:
[(H.sub.2O).sub.n+H].sup.++Sample->[Sample
+H].sup.++nH.sub.2O
[0007] Many compounds are ionized under these conditions. However,
some compounds are not efficiently ionized (for example, alkanes)
because they do not have a higher proton affinity than water or
water clusters. A compound will only accept a proton if it has a
higher PA than the donor.
[0008] Direct ionization of an analyte by oxygen charge-exchange
ionization or chemical ionization with nitric oxide (NO.sup.+) has
been reported, but only for ion sources operating in a vacuum or
under reduced-pressure conditions. It has not been employed as a
positive-ion formation mechanism for atmospheric pressure ion
sources.
[0009] Fluorobenzene has been used as a dopant to promote the
formation of molecular ions (M.sup.+) by charge exchange in
atmospheric pressure photoionization (APPI) with the use of a
high-intensity lamp (10 eV).
SUMMARY OF THE INVENTION
[0010] Briefly, according to one embodiment of the present
invention, there is provided a method of producing analyte, analyte
fragment, and/or analyte adduct ions at atmospheric pressure for
mass spectrographic analysis from specimens having a proton
affinity less than the proton affinity of water and water clusters.
The method comprises the steps of introducing a carrier gas at
atmospheric pressure into a chamber and adding energy to the
chamber creating metastable neutral excited-state species and
directing the carrier gas and metastable species at atmospheric
pressure into contact with the specimen to form analyte ions,
analyte fragment ions, and/or analyte adduct ions under conditions
that suppress the formation of protonated water clusters and
promote charge-exchange ionization. The suppression of the
formation of protonated water clusters enables other ionization
mechanisms, such as charge exchange with oxygen chemical ionization
by nitric oxide and direct Penning ionization.
[0011] The conditions for adding energy to the chamber may comprise
establishing an electrical potential difference between electrodes,
photo excitation, microwave excitation or dielectric barrier
discharge (one or both electrodes covered with dielectric layers).
The conditions are selected to create metastable neutral
excited-state species of the carrier gas.
[0012] According to another embodiment of the present invention,
the carrier gas and metastable species are directed from the
chamber into a reactant gas at atmospheric pressure, wherein the
metastable species interact with the reactant gas to produce ions
of the reactant gas under conditions that suppress the formation of
protonated water clusters and promote charge-exchange ionization.
The carrier-gas/reactant-gas/ionized-derivative mixture is directed
into contact with the specimen maintained at atmospheric pressure
and near ground potential.
[0013] It is an advantage, according to the present invention, that
the analyte may be gaseous or non-volatile and that the analyte may
be ionized at a liquid or solid surface.
[0014] It is a further advantage, according to the present
invention, that specimens with a proton affinity less than water or
water clusters can be ionized.
[0015] It is a still further advantage, according to the present
invention, to charge-exchange ionized specimens by charge exchange
with oxygen ions [O.sub.2.sup.+.cndot.] such that the mass spectrum
produced is similar to spectra produced with vacuum-based electron
impact (EI) ionization.
[0016] It is a still further advantage, according to the present
invention, to ionize specimens by chemical ionization with the
nitric oxide ions [NO.sup.+.cndot.].
[0017] According to a preferred embodiment of the present
invention, there is provided a method for atmospheric pressure
ionization comprising: into a atmospheric pressure chamber
introducing a carrier gas between a first electrode and a
counter-electrode for creating a corona or glow electric discharge
in the carrier gas causing the formation of neutral excited-state
metastable species, and directing the carrier gas from the chamber
into a reactant gas, for example, room atmosphere, maintained at
atmospheric pressure under conditions to minimize formation of
protonated water clusters and to form intermediate ionized species
in a mixture of the carrier gas and reactant gas and directing the
mixture of carrier gas and reactant gas into contact with a
specimen maintained at atmospheric pressure and near ground
potential to faun analyte ions, analyte fragment ions and/or
analyte adduct ions.
[0018] In an apparatus for practicing the methods disclosed herein,
a first electrode and counter-electrode must be maintained at
potentials sufficient to induce an electrical discharge. The
counter-electrode also serves to filter ionized species formed in
the discharge. The potential difference between the first electrode
and counter-electrode necessary for the formation of a discharge
depends on the carrier gas and the shape of the first electrode and
is usually several hundreds of volts, say 400 to 1,200. The first
electrode, for example, a needle electrode, may have either a
positive or negative potential. The counter-electrode is normally
grounded or of polarity opposite to the needle electrode. This is
the case whether operating in the positive ion or negative ion
mode. In the positive ion mode, a lens electrode may be between
ground potential and a few hundred positive volts to filter out
negative ions in the carrier gas. Also, in the negative ion mode, a
lens electrode may be between ground and minus a few hundred volts
to filter out positive ions in the carrier gas.
[0019] According to another embodiment of the present invention,
the carrier gas may be heated prior to introduction into the
discharge or thereafter to facilitate vaporization or desorption of
the analyte into the gas phase from surfaces and/or
fragmentation.
[0020] By atmospheric pressure in this specification and the
appended claims is meant pressures near ambient pressures, say 400
to 1,400 Torr. This would include pressurized aircraft and
submerged submarines. For laboratory use, typical ambient pressures
may fall within the range 700 to 800 Torr. By ambient temperature
in the specification and claims is meant temperatures between
0.degree. and 50.degree. C., i.e., temperatures that may be
encountered in living and working environments.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] Other objects and features of the invention will appear in
the course of the description thereof, which follows:
[0022] FIG. 1 is a perspective view of an atmospheric pressure
interface or device useful, according to the present invention;
[0023] FIG. 2 is a broken away perspective view similar to FIG.
1;
[0024] FIG. 3 is a detail from the perspective view of FIG. 2;
[0025] FIG. 4 is a schematic circuit diagram of a power supply for
an atmospheric pressure device or source useful, according to the
present invention;
[0026] FIGS. 5A and 5B display comparative mass spectra of
background ions for atmospheric ionization with neutral
excited-state species without suppression of water clusters
(proton-transfer ionization) and with suppression of protonated
water clusters (charge-exchange ionization);
[0027] FIGS. 6A, 6B and 6C display mass spectra of n-Hexadecane for
atmospheric ionization with neutral excited-state species without
suppression of protonated water and water clusters (proton transfer
ionization), with suppression of protonated water and water
clusters (charge-exchange ionization), and for comparison electron
ionization in a conventional vacuum source;
[0028] FIGS. 7A, 7B and 7C display mass spectra of cholesterol as
determined with atmospheric pressure ionization with water clusters
(proton transfer ionization), with the use of fluorobenzene dopant,
and with water cluster suppression and charge-exchange
ionization;
[0029] FIGS. 8A and 8B display mass spectra of Hexadecane by
charge-exchange ionization at two gas temperatures illustrating the
temperature effect on fragmentation; and
[0030] FIGS. 9A and 9B display two GC/MS chromatograms of a test
mix of Grob gas (atmospheric ionization with neutral excited-state
species with and without suppression of protonated water
clusters).
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0031] Referring to FIGS. 1 to 3, an apparatus useful for practice
of this method invention consists of a tube divided into several
chambers through which a gas, such as nitrogen or helium, is
allowed to flow. The gas is introduced into a discharge chamber
where an electrical potential is applied between a discharge needle
at kilovolt potentials and a perforated counter-electrode held at
ground potential. A plasma consisting of ions, electrons, and
excited-state species is produced in the discharge region. The gas
is allowed to flow into an optional second chamber where a second
perforated electrode can be biased to remove ions from the gas
stream. The gas flow passes through an optional third region that
can be optionally heated. Gas exits through an optional third
perforated electrode or grid and is directed toward the mass
spectrometer sampling orifice. The grid serves two functions: it
acts as an ion repeller and it serves to remove charged species of
the opposite polarity thereby preventing signal loss by
ion-electron recombination. The gas flow can be directed toward a
liquid or solid sample or it can interact with vapor-phase
samples.
[0032] A typical reaction sequence wherein steps are not taken to
prevent the formation of charged water clusters is shown below
using helium to form the initial excited-state molecules.
[0033] 1. Formation of excited-state atoms of molecules
(e.sup.-=electron):
He.sup.0(electrical discharge).fwdarw.He.sup.+.cndot.+e.sup.-
He.sup.+.cndot.+He.sup.*
[0034] The He.sup.*3S1 state has an energy of 19.8 eV which is
above the ionization potential of water of 12.6 eV.
[0035] 2. Formation of charged water clusters:
He.sup.*+H.sub.2O.fwdarw.H.sub.2O.sup.+.cndot.+He+e.sup.-
H.sub.2O.sup.+.cndot.+H.sub.2O.fwdarw.H.sub.3O.sup.++OH.sup..cndot.
H.sub.3O.sup.++(H.sub.2O).sub.n.fwdarw.[(H.sub.2O).sub.nH].sup.+
[0036] 3. Reaction of charged water clusters to ionize target
analyte molecule M:
[(H.sub.2O).sub.nH].sup.++M.fwdarw.[M+H].sup.++(H.sub.2O).sub.n
[0037] If helium is used as the carrier gas, the principal
excited-state species has an energy of 19.8 eV. This energy is
sufficient to ionize most molecules. Under normal conditions, the
excited-state helium rapidly reacts with atmospheric moisture to
produce positive-ion water clusters or negative-ion clusters
containing oxygen and water. The reaction between excited-state
helium and water molecules is extremely rapid. Under these
conditions, the primary mode of ionization is proton transfer from
the ionized water clusters. The largest peaks in the background
spectrum are water clusters [(H.sub.2O).sub.nH].sup.+ formed by
interaction of excited-state helium atoms with the sample.
[0038] Water has a proton affinity (PA) of 691 kJ/mol. Proton
transfer occurs if the sample has a higher proton affinity than the
PA of the water clusters. Many compounds are ionized under these
conditions. However, some compounds (e.g., alkanes) are not
efficiently ionized because they do not have a higher proton
affinity than water or water clusters. A compound will only accept
a donated proton if it has a higher PA than the donor.
[0039] If the conditions are modified to inhibit formation of
protonated water clusters, the primary mode of ionization can be
changed to a combination of proton transfer and charge exchange
from oxygen ions, for example:
O.sub.2.sup.+.cndot.+Sample->Sample.sup.+.cndot.O.sub.2
[0040] Direct Penning ionization may also occur when protonated
water clusters are suppressed as follows:
He*+Sample->Sample.sup.+.cndot.+electron.sup.-+He
[0041] FIGS. 5A and 5B display comparative mass spectra of
background ions for atmospheric ionization with neutral
excited-state species with and without suppression of protonated
water clusters. The major peaks observed under normal conditions
are water clusters and ammonium. The major peaks when protonated
water clusters are suppressed are water clusters and
O.sub.2.sup.+.cndot.. The relative abundance of
O.sub.2.sup.+.cndot. and [(H.sub.2O).sub.nH].sup.+ can be varied
depending on gas flow, humidity, the exit position of the source of
neutral excited-state species relative to the intake orifice of the
mass spectrometer, and the potential on the grid at the exit
position of the source neutral excited-state species. The small
unlabeled peaks in the background of FIG. 5A are the result of
solvent vapor (methanol, ethanol, acetone) present in the
laboratory air.
[0042] The ionization potential (IP) of oxygen (O.sub.2) is 12.07
eV, which is higher than the IP for most common organic compounds
including alkanes. For charge-exchange ionization, a compound will
only accept a donated electron if it has a lower IP than the
donor.
[0043] Mass spectra obtained under these conditions for alkanes
look very much like electron ionization (EI) mass spectra,
including characteristic fragment ions that are used for compound
identification by database searching. Molecular ions M.sup.+.cndot.
are observed and [M-H].sup.+ may be observed.
[0044] FIGS. 6A, 6B and 6C display mass spectra of n-Hexadecane for
atmospheric ionization with neutral excited-state species without
suppression of protonated water and water clusters, with
suppression of protonated water and water clusters and electron
ionization in a conventional vacuum source. The mass spectrum shown
in FIG. 6B yields the correct identification of the sample when
compared to the databases for EI ionization, whereas a database
search on the mass spectrum of FIG. 6A did not.
[0045] Aromatic hydrocarbons with electron ionization (EI)
uniformly produce molecular ions M.sup.+.cndot. and proton transfer
ions [M+H].sup.+. With water cluster ionization at atmospheric
pressure, these ions are not always produced. With ionization by
charge exchange, from oxygen ions these ions are produced. This
mode of ionization has other useful characteristics. The chemical
background is reduced making it easier to recognize changes in the
ion current when an analyte is present. Furthermore, ion efficiency
is more uniform for compounds with different functional groups.
[0046] An advantage of the open-air charge-exchange method is that
a mass spectrum similar to that obtained by EI can be obtained
without the drawback of EI vacuum-based sources. In particular, the
electron filaments used in EI are fragile and can break if exposed
to air or oxygen while hot. They must be periodically replaced. The
open-air charge-exchange method does not require a replaceable
filament.
[0047] FIGS. 7A, 7B and 7C display mass spectra of cholesterol as
determined with atmospheric pressure proton transfer ionization
from water clusters; oxygen charge exchange ionization; and
fluorobenzene dopant charge-exchange ionization. Charge-exchange
ionization has been shown to be effective for producing molecular
ions from cholesterol. Fluorobenzene has a proton affinity of 775.9
kJ/mol and an ionization potential of 9.2 eV. Hence, it will react
by charge exchange to produce molecular ions as analytes with an IP
less than 9.2 eV. Proton transfer, a seen in FIG. 7A, produces an
abundant [M+H-H.sub.2O].sup.+ peak, but no molecular ion. The
charge-exchange method is clearly superior. Fluorobenzene has been
used as a dopant for atmospheric pressure photoionization (APPI)
with the use of a high-intensity lamp (10 eV). The charge-exchange
process, according to the present invention, may be assisted by
pulsed photon desorption to produce molecular ions from
low-volatility compounds.
[0048] FIGS. 8A and 8B display mass spectra of Hexadecane at two
gas temperatures illustrating the temperature effect on
fragmentation. The relative abundance of molecules and fragment
ions depends on gas temperature. At relatively low temperatures
(temperatures required to desorb or vaporize the sample, for
example, subambient up to about 200.degree. C.), the molecular ion
is abundant and the fragment ions are of low abundance. The
relatively high abundance of M.sup.+.cndot. and [M-H].sup.+ makes
it easy to identify the molecular weight of the sample.
Fragmentation increases with increasing gas temperature with
fragment ions becoming dominant at gas temperatures in the range
200.degree. to 300.degree. C. or higher. Under these conditions,
the mass spectrum of an n-alkane is virtually identical to a
conventional EI mass spectrum with the exception that a [M-H].sup.+
peak may be observed. As with EI, the presence of fragments is a
"fingerprint" facilitating identification with database searching
and often permits distinguishing isometric compounds.
[0049] A temperature ramp (programming the carrier gas temperature
from low to high in a time-dependent manner) can be used to
separate compounds according to their desorption temperature. In
this way, an abundant molecular ion can be observed for both
high-volatility and low-volatility compounds in a given sample or
specimen. This has been demonstrated with a mixture of n-alkanes
with carbon numbers from C6 to C44. Abundant molecular ions with
minimal fragmentation could be observed for all compounds.
[0050] FIGS. 9A and 9B display two GC/MS chromatograms of a test
mix of Grob gas. The gas chromatograph column separates the
compounds and the MS is used to identify the separated compounds.
The output of the chromatograph was directed to the output of the
source of excited-state neutral carrier gas. In the case of FIG.
9A, the formation of protonated water clusters was suppressed to
promote charge-exchange ionization. In the case of FIG. 9B, it was
not. (Note that a slower GC oven temperature program was used for
the analysis depicted in FIG. 9A than for FIG. 9B. This will change
the retention times of components, but will not affect the elution
order signal-to-noise ratio.) Alkanes, e.g., decane and undecane,
were not detected when protonated water clusters were not
suppressed.
[0051] When suppressing the formation of protonated water clusters,
a certain amount of NO.sup.+ (nitric oxide ion) is observed. Nitric
oxide is a well-known chemical ionization reagent for chemical
ionization of alkanes and aromatic hydrocarbons. The ionization
mechanism may be charge-exchange producing M.sup.+ or
hydride-abstraction producing [M-H].sup.+ ions. Nitric oxide
adducts [M+NO].sup.+ can also be observed for aromatic compounds.
Nitric oxide chemical ionization can also result in oxidation of
the analyte. Other reaction processes can occur when operating with
a nitrogen carrier gas to ionize alkanes and aromatics. Oxygen can
be incorporated into the molecule, producing abundant oxidized
species, such as [M+O-3H].sup.+ and [M+O.sub.2-H].sup.+ from
ionization of alkanes.
[0052] The carrier gases that have been used are helium and
nitrogen. Any gas or mixture of gases with a metastable state lying
higher than a state of the analyte is a potential carrier gas. Both
helium and nitrogen have high first electron ionization potentials
and are not reactive with other elements or compounds at room
temperature and pressure.
[0053] The atmospheric-pressure ionization method described herein
is useful for the introduction of ions into mass spectrometers and
ion mobility spectrometers for the detection and identification of
analytes of interest, such as drugs, explosives, chemical weapons,
toxic industrial materials and the like. This method is
non-radioactive and provides rapid sampling of gas and vapor in
headspace sampling. It also permits rapid and direct sampling of
chemicals on surfaces.
[0054] Referring again to FIG. 1, a physical implementation of an
atmospheric-pressure ion device useful, according to the present
invention, may comprise a tubular non-conductive casing 10 which
may be fabricated from a Teflon.RTM.-type plastic (good temperature
resistance), glass, a ceramic material or other non-conductive
material. Extending from one end of the casing 10 is a disposable
glass tube insert 11 with a non-conductive end piece 13 that serves
to hold a mesh electrode or grid 14 in place. The mesh electrode 14
is connected by an insulated wire 15 to a micro jack 17 on the
casing 10. At the opposite end of the casing 10 is a carrier gas
inlet comprising a connector 18 with a corrugated surface for
holding a flexible tube slide thereon. Micro jacks 21, 22, 23, and
24 are threaded in the casing for connecting leads from a power
supply to the various electrodes within the casing 10.
[0055] Referring now to FIG. 2, the interior of the casing is
divided into first and second chambers. At each axial end, a hollow
plug is fixed to the casing. At the inlet end, a plug 26 has
threads for receiving the inlet connector 18. At the outlet end, a
plug 27 is provided with interior annular grooves for receiving
Viton O-rings 38 that seal against the exterior surface of the
glass tube insert 11. Non-conductive spacer 30 holds the needle
electrode 31 which is connected to micro jack 21 and defines a
first chamber in which a corona or glow electrical discharge is
created. A conductive spacer and electrode baffle 32 are positioned
within the casing and adjacent to the non-conductive spacer
supporting the needle. The conductive spacer 32 is connected to
micro jack 23. A non-conductive spacer 33 is positioned within the
casing and is adjacent to the conductive spacer 32 to define a
second chamber. Another conductive spacer and electrode baffle 34
are positioned adjacent to the non-conductive spacer 33 to define
the axial outlet end of the second chamber. The conductive spacer
34 abuts the glass tube insert 11. This conductive spacer is
connected to micro jack 22. The micro jack 24 is in communication
with an electrical conduit that runs axially to the outlet end of
the casing where it connects to the micro jack 17.
[0056] Referring to FIG. 3, the end of the glass tube with the
non-conductive end piece 13 is shown in more detail. The
non-conductive end piece 13 spaces the grid from direct contact
making it difficult to come into contact with the high voltage on
the grid. The hole in the end piece allows the escape of the
excited-state gas to ionize the analyte. A copper washer 39 abuts
the end of the glass tube and is soldered to insulated wire lead
15. Held against the washer is a grid electrode 14. The hollow
glass tube 11 and grid electrode 14 define a third chamber.
[0057] Referring to FIG. 4, an example of a power supply for an
atmospheric pressure ion source is shown schematically. AC current
passes switch S.sub.1 and fuse F.sub.1 and is applied to switcher
power supply SPS. The 15 volt DC output is applied across filter
capacitor C.sub.1 to current regulator CR. The regulated current is
applied across filter capacitor C.sub.2 to the high-voltage direct
current converter DC-HVDC. The high voltage of this device is
applied through current limit resistor R.sub.1 to the electrode for
creating a corona or glow discharge. The 15 volt output is also
applied to a plurality of general purpose, high-current positive
voltage regulators VR. The output of the voltage regulators is
applied across filter capacitor C.sub.3 to pass current to
high-voltage direct converters DC-HVDC.sub.2. The output of the
converters is applied to potentiometers R.sub.7 enabling adjustment
of the potential on the lens electrodes. Those skilled in power
supply design will understand how to configure a circuit for
negative output potentials.
[0058] The techniques currently found to suppress formation of
protonated water and water cluster ions are a) increasing the
potential of the exit grid electrode from about 150 V to about 500
to 600 V or greater, b) moving the hole in the end piece of the
source to within about 5 mm or less of the inlet port of the mass
spectrometer, c) sweeping the sample with desiccated air or oxygen
of fluorobenzene or anisole or a suitable dopant, d) heating the
apparatus to bake out residual moisture before operating, or e) any
combination of these techniques. In the case of GC/MS experiments,
the outlet of the GC column or gas transfer line is connected to an
extension of the apparatus above described and the outlet port of
the extension tube is placed at the sampling orifice of the mass
spectrometer atmospheric-pressure interface. The extension tube
isolates the carrier gas/neutral excited-state mixture from
atmospheric moisture and permits the formation of the reagent ions
[O.sub.2.sup.+.cndot.], for example, formed by leaking trace
reagent gases into the loosely-sealed tube. The present invention
is not tied to any particular technique for preventing or
suppressing the formation of protonated water and water molecules
in the vicinity of the sample. Complete suppression is not
essential so long as an adequate quantity of charge-exchange ions
and/or Penning electrons are formed and directed to the sample.
[0059] The atmospheric-pressure ionization method described herein
is useful for the introduction of ions into mass spectrometers and
ion mobility spectrometers or hybrid ion-mobility spectrometer-mass
spectrometer for the detection and identification of analytes of
interest, such as drugs, explosives, chemical weapons, toxic
industrial materials and the like. It is non-radioactive and
provides rapid sampling of gas and vapor in headspace sampling. It
also permits rapid and direct sampling of chemicals on surfaces.
This feature makes the ion source described herein a very useful
replacement for a radioactive source on IMS detectors.
[0060] Having thus described my invention in the detail and
particularity required by the Patent Laws, what is desired
protected by Letters Patent is set forth in the following
claims.
* * * * *